Ground state phase diagram of a spin-2 antiferromagnet on the square lattice

We use the vertex state model approach to construct optimum ground states for a large class of quantum spin-2 antiferromagnets on the square lattice. Optimum ground states are exact ground states of the model which minimize all local interaction operators. The ground state contains two continuous parameters and exhibits a second order phase transition from a disordered phase with exponentially decaying correlation functions to a Néel ordered phase. The behaviour is very similar to that of the corresponding ground state of a quantum spin-3/2 model on the hexagonal lattice, which has been investigated in an earlier paper. Received 8 April 1999     

一维Fibonacci准周期Frustrated Ising模型的热力学特性

分别给出周期和自由边界条件下求解一维Fibonacci准周期Frustrated Ising模型的配分函数的方法。研究它的低温热力学性质,发现当温度趋于零时,其热力学函数在参数b/J=2/(2m+1)(m为正整数)处发生尖锐的变化。分析了零温时系统在不同参数范围内的基态构形,对计算的结果进行了解释。 关键词：     

Aperiodic crystals: A contradictio in terminis?

Although in the prevailing view a necessary condition for having a crystalline phase is lattice periodicity, it has become clear in the last decades that there are physical systems with many properties of the usual crystalline state but without three-dimensional lattice periodicity. Incommensurate modulated crystals have been known now for some time, and a couple of years ago much excitement was raised by the discovery of quasicrystals, systems with long-range order but with five-fold symmetry axes, which exclude lattice periodicity.A discussion is given of the various generalizations of the concept of lattice periodicity. In fact, these go from ordinary periodic crystal st structures to almost chaotic ones. One of these is the notion of quasiperiodicity. Section two deals with a special type of these quasiperiodic systems, tilings or space fillings with tiles or blocks of a small number of types. In section three the symmetry of quasiperiodic systems is discussed. Here the embedding into a higher-dimensional space is the key concept. Section four deals with N-dimensional crystallographic groups that occur as symmetry groups of quasiperiodic systems, so called superspace groups. In section five the diffraction from quasiperiodic systems is treated, and in section six it is shown that in some cases quasiperiodic structures may be approximated by periodic ones, and that periodic systems sometimes are more conveniently described by quasiperiodic ones. The emphasis in the symmetry discussion is on quasicrystals.This is even more so in the remaining sections. Section seven gives a brief account of the many experimental data, section eight describes what is known about the microscopic structure. Imperfections are even more important for quasiperiodic systems than for periodic ones. They are discussed in section nine.Not only microscopically do quasiperiodic systems have similarities with ordinary crystals, but also macroscopically. The morphological laws may be generalized to quasiperiodic systems, as shown in section ten. The consequences of quasiperiodicity on the physical properties is still to a large extent unclear. Mathematically they differ much from periodic systems. A discussion of a number of results is given in section eleven.     

Metamagnetism

This is a review of the physical properties of metamagnets. These are antiferromagnets which, upon the application of a magnetic field, can undergo first-order magnetic phase transitions to a state with a relatively large magnetic moment. The treatments of mean field theory describing these materials are reviewed, as are the treatments of more modern theories. The experimental properties of the known metamagnets are discussed, with emphasis on the variety of means by which the metamagnetic transitions have been observed and studied. For some materials, there have been studies of the tricritical behaviour, and a discussion of the experimental results of these studies is given, along with a comparison of the results with the present theory.     

Quantum spins and quasiperiodicity: a real space renormalization group approach

We study the antiferromagnetic spin-1/2 Heisenberg model on a two-dimensional bipartite quasiperiodic structure, the octagonal tiling, the aperiodic equivalent of the square lattice for periodic systems. An approximate block spin renormalization scheme is described for this problem. The ground state energy and local staggered magnetizations for this system are calculated and compared with the results of a recent quantum Monte Carlo calculation for the tiling. It is conjectured that the ground state energy is exactly equal to that of the quantum antiferromagnet on the square lattice.     

Magneto-optics of antiferromagnets

Antiferromagnetic crystals in which crystallographic sites occupied by magnetic ions from various sublattices are not transnationally equivalent and are not associated with each other by a symmetry center can have magneto-optic properties distinct from the properties of other antiferromagnets. In particular, birefringence and dichroism of linear polarized light can be observed which are directly proportional to the magnetic field strength, as well as magnetic rotation and circular dichroism quadratic in the field strength. Both effect—the linear magneto-optic effect and quadratic magnetic gyration — are sensitive to the crystal magnetic symmetry and to reorientation of the antiferromagnetic vector. Both effects reverse their signs when the directions of the magnetic moments of a sublattice are changed. These properties of new magneto-optic effects can be used to study the time-reversed domain structure of antiferromagnets, to define the symmetry of magnetic ordering and to study the magnetic crystal energy spectra by spectroscopic methods. The results of experimental studies of the linear magneto-optic effect and quadratic magnetic rotation in tetragonal antiferromagnetic fluorides of transition metals, manganese-germanium garnet and other antiferromagnets are reported. Experimental results on the domain structure of high symmetric antiferromagnets, the point magnetic symmetry of non-collinear multisublattice antiferromagnetic garnet MnGeG are discussed.     

Hadron model with braking of spatial homogeneity of vacuum

A possibility of breaking spatial homogeneity of vacuum as a result of interaction of a quark with a Bose field has been investigated. It is shown that there can be in a ground state of the wave-packet type. The energetic conditions for such a state have been found. The origin of deep inelastic processes and the phenomenon of quark confinement are discussed.     

Quantum antiferromagnetism in quasicrystals

The antiferromagnetic Heisenberg model is studied on a two-dimensional bipartite quasiperiodic lattice. Using the stochastic series expansion quantum Monte Carlo method, the distribution of local staggered magnetic moments is determined on finite square approximants with up to 1393 sites, and a nontrivial inhomogeneous ground state is found. A hierarchical structure in the values of the moments is observed which arises from the self-similarity of the quasiperiodic lattice. The computed spin structure factor shows antiferromagnetic modulations that can be measured in neutron scattering and nuclear magnetic resonance experiments. This generic model is a first step towards understanding magnetic quasicrystals such as the recently discovered Zn-Mg-Ho icosahedral structure.     

Phase transition in aperiodic spin chains

The quantum-mechanical transition in the ground state of some aperiodic spin chains is considered. Exact expressions for the energy gap and the dispersion relation of low-energy excitations close to the critical point are derived. Applications of these results to the study of particular quasiperiodic and random models are discussed.     

Zero-field incommensurate spin-Peierls phase with interchain frustration in TiOCl

We report on the magnetic, thermodynamic, and optical properties of the quasi-one-dimensional quantum antiferromagnets TiOCl and TiOBr, which have been discussed as spin-Peierls compounds. The observed deviations from canonical spin-Peierls behavior, e.g., the existence of two distinct phase transitions, have been attributed previously to strong orbital fluctuations. This can be ruled out by our optical data of the orbital excitations. We show that the frustration of the interchain interactions in the bilayer structure gives rise to incommensurate order with a subsequent lock-in transition to a commensurate dimerized state. In this way, a single driving force, the spin-Peierls mechanism, induces two separate transitions.     

Classical lattice gas model with a unique nondegenerate but unstable periodic ground state configuration

We construct a classical lattice gas model with a unique periodic ground state configuration such that the Peierls' condition is not satisfied. The ground state configuration is nondegenerate, which means that for any fixed energyE and any integern, the diameter of the support of alln-connected local excitations, with energy less thanE, is bounded. Nevertheless the configuration is not stable: it does not give rise to a low temperature phase. Any translation invariant Gibbs state of our model corresponds to quasiperiodic ground state configurations. This requires the modification of a recent hypothesis of Dobrushin and Shlosman.     

Relativistic Mean-Field Study to Odd-A Nuclei Far from Stability

Ground state properties of recently discovered odd-A nuclei near the particle-dripline have been investigated in the relativistic mean-field model. Special emphasis is placed on the effect of the spatial component of vector meson fields on ground state properties which is due to breaking of time reversal invariance for odd-A nuclei. Calculations show that its contribution to binding energies, radii and single particle energies is small but cannot be neglected completely for some drip-line nuclei because the last nucleon in them is very weakly bound. Calculated binding energies are in reasonably good agreement with present experimental data and estimated values in mass tables. The structure of newly discovered nuclei is also discussed.     

Perspectives of antiferromagnetic spintronics

Antiferromagnets are promising for future spintronic applications owing to their advantageous properties: They are magnetically ordered, but neighboring magnetic moments point in opposite directions, which results in zero net magnetization. This means antiferromagnets produce no stray fields and are insensitive to external magnetic field perturbations. Furthermore, they show intrinsic high frequency dynamics, exhibit considerable spin–orbit and magneto-transport effects. Over the past decade, it has been realized that antiferromagnets have more to offer than just being utilized as passive components in exchange bias applications. This development resulted in a paradigm shift, which opens the pathway to novel concepts using antiferromagnets for spin-based technologies and applications. This article gives a broad perspective on antiferromagnetic spintronics. In particular, the manipulation and detection of antiferromagnetic states by spintronics effects, as well as spin transport and dynamics in antiferromagnetic materials will be discussed. We will also outline current challenges and future research directions in this emerging field.     

Ground States of Quantum Antiferromagnets in Two Dimensions

We explore the ground states and quantum phase transitions of two-dimensional, spin S=1/2, antiferromagnets by generalizing lattice models and duality transforms introduced by Sachdev and Jalabert (1990, Mod. Phys. Lett. B4, 1043). The minimal model for square lattice antiferromagnets is a lattice discretization of the quantum nonlinear sigma model, along with Berry phases which impose quantization of spin. With full SU(2) spin rotation invariance, we find a magnetically ordered ground state with Néel order at weak coupling and a confining paramagnetic ground state with bond charge (e.g., spin Peierls) order at strong coupling. We study the mechanisms by which these two states are connected in intermediate coupling. We extend the minimal model to study different routes to fractionalization and deconfinement in the ground state, and also generalize it to cases with a uniaxial anisotropy (the spin symmetry groups is then U(1)). For the latter systems, fractionalization can appear by the pairing of vortices in the staggered spin order in the easy-plane; however, we argue that this route does not survive the restoration of SU(2) spin symmetry. For SU(2) invariant systems we study a separate route to fractionalization associated with the Higgs phase of a complex boson measuring noncollinear, spiral spin correlations: we present phase diagrams displaying competition between magnetic order, bond charge order, and fractionalization, and discuss the nature of the quantum transitions between the various states. A strong check on our methods is provided by their application to S=1/2 frustrated antiferromagnets in one dimension: here, our results are in complete accord with those obtained by bosonization and by the solution of integrable models.     

A phenomenological theory of metastable states in disordered Ising magnets

A mechanism of the formation of an exponentially large number of metastable states in magnetic phases of disordered Ising magnets as a result of condensation of fractal delocalized modes near the localization threshold is suggested. The thermodynamic properties of metastable states are studied in the effective-field approximation in the vicinity of transitions in magnets with zero uniform magnetization in the ground state such as dilute antiferromagnets, spin glasses, and dilute ferromagnets with dipole interaction. These properties are shown to determine the parameters of nonequilibrium processes in the glassy phase, namely, the shape of the hysteresis loop, the thermodynamic values in field-cooled and zero-field-cooled regimes, and the thermoremanent and isothermal remanent magnetization values.     

Stable Quasicrystalline Ground States

We give strong evidence that noncrystalline materials such as quasicrystals or incommensurate solids are not exceptions, but rather are generic in some regions of phase space. We show this by constructing classical lattice-gas models with translation-invariant finite-range interactions and with a unique quasiperiodic ground state which is stable against small perturbations of two-body potentials. More generally, we provide a criterion for stability of nonperiodic ground states.     

铝原子团簇Al5、Al5-和Al5+稳定结构的密度泛函理论研究

采用密度泛函理论的四种方法 :杂化密度泛函B3LYP与B3PW 91、Perdew Wang91交换与相关泛函WP91PW91、局域自旋密度近似SVWN ,研究了Al5、Al5-和Al5+ 团簇的多种可能结构 ,找到了它们稳定的结构与自旋态 ,与已有的理论结果作了比较 ,并计算了Al5-的绝热与垂直电子离解能、Al5的绝热与垂直电离势 ,同有关的实验数据比较 ,符合较好 .同时对四种密度泛函方法的计算结果作了一些比较与讨论     

Decagonal quasiferromagnetic microstructure on the penrose tiling

The stable magnetization configurations of a ferromagnet on a quasiperiodic tiling have been derived theoretically. The magnetization configuration is investigated as a function of the ratio of the exchange to the dipolar energy. The exchange coupling is assumed to decrease exponentially with the distance between magnetic moments. It is demonstrated that for a weak exchange interaction the new structure, the quasiferromagnetic decagonal configuration, corresponds to the minimum of the free energy. The decagonal state represents a new class of frustrated systems where the degenerated ground state is aperiodic and consists of two parts: ordered decagon rings and disordered spin-glass-like phase inside the decagons.     

量子点中束缚极化子性质的温度依赖性

采用线性组合算符和幺正变换方法研究抛物量子点中弱耦合束缚极化子性质的温度依赖性,导出了弱耦合束缚极化子的振动频率、基态能量和声子平均数随温度的变化关系。取ZnS晶体为例进行数值计算,结果表明:量子点中弱耦合束缚极化子的振动频率、基态能量和声子平均数随温度的升高而增大。     

Effect of positional substitution of amino group on excited state dipole moments of quinoline

The effect of positional substitution of amino group on the ground and excited state dipole moments of quinoline ring has been investigated using solvatochromic shift methods. The excited state dipole moments of 5aminoquinoline (5AQ) and 3aminoquinoline (3AQ) have been estimated from the spectral data in different non-polar, polar aprotic and polar protic solvents using Bakhshiev and Kawski-Chamma-Viallet equations. It has been observed that both grounds as well as excited state dipole moments for 5AQ are higher than those for 3AQ by approximately a factor of two. Higher values of the excited state dipole moments for both 3AQ and 5AQ as compared to corresponding ground state values have been attributed to intramolecular charge transfer processes. The role of specific solute-solvent interaction on excited state dipole moment in addition to the general solvent effects has been discussed.